THE SKELETON OF THE DOG. THE SKULL. 



391 



These ossifications however very early combine to form a 

 single bone, the periotic, which nevertheless consists of two 



FIG. 74. 



DIAGRAM OF THE MAMMALIAN TYMPANIC CAVITY AND ASSOCIATED 

 PARTS (modified from LLOYD MORGAN). 



1. external auditory meatus. 



2. tympanic membrane. 



3. malleus. 



4. incus. 



5. lenticular. 



6. stapes. 



7. fenestra ovalis. 



8. fenestra rotunda. 



9. Eustachian tube. 



10. cavity occupied by the cochlea. 



11. cavity occupied by the mem- 



branous labyrinth. 



portions, the petrous and the mastoid, differing considerably 

 from one another. 



The petrous portion lies dorsally and anteriorly, and is 

 much the more important of the two, as it encloses the 

 essential part of the auditory organ. It forms an irregular 

 mass of hard dense bone, projecting into the cranial cavity, 

 and does not appear on the external surface at all. The 

 mastoid portion lies ventrally and posteriorly, is smaller, 

 and formed of less dense bone than is the petrous portion, 

 from which it differs also in the fact that it appears on the 

 surface of the skull, just external to the exoccipital. The 

 petrous portion bears a ridge, which together with a ridge 



